Process and apparatus for applying alternating positive and negative loads



May 25, 1937. X 2,081,404

PROCESS AND APPARATUS FOR APPLYING ALTERNATING POSITIVE AND NEGATIVE LOADS Filed June 20, 1935 5 Sheets-Sheet l May 25, 1937. w, ARX A 2,081,404

PROCESS AND APPARATUS FOR APPLYING ALTERNATING POSITIVE AND NEGATIVE LOADS Filed June 20 1955 3 Sheets-Sheet 2 Elma/rm May 25, 1937. w, MARX 2,081,404

PROCESS AND APPARATUS FOR APPLYING ALTERNATING POSITIVE AND NEGATIVE LOADS Filed June 20, 1935 3 Sheets-Sheet 3 my Marx Patented May 25, 1937 nocnss AND APPARATUS FOR APPLYING ALTERNATINGMPOSITIYE AND' NEGATIVE.

LOADS Wilhelm Marx, Dusseldorf, Germany, assignor to Losenhausenwerk, DusseldorfeGrafenberg,

Germany Application June 20, 1935, Serial No. 27,547 In Germany February 12, 1931 .12 Claims.

There have already been known devices for applying loads to a test piece alternately in Onposite directions, for example loads: changing from tension to compression. In these known de 55? vices .a special driving means, for example a so called pulsator, is provided for each direction in which the load is applied; for instance, there is a driving means for exerting tension and .another for compression. The two pulsators 510 are coupled together in a suitable way to bring .about the alternate application of the two kinds of loads. The use of two pulsation devices makes the machine relatively complicated and expensive.

The present invention relates to a process and apparatus for exerting alternating loads, in which only a :singledrivingmeans is required. In accordance with the invention the process starts with. a definite static initial load applied through 20 a suitableelastic force transmitting mechanism, against which periodically varying counterforces are exerted by means .of a pulsation device, which maybe of known form. In this way the test piece is subjected to a net load equal to the instantaneous difference between the initial stress and the counterforce, which net load can be made to change from one direction to the other by suitable adjustment of the forces.

The drawings show several forms of the invention.

Fig. 1 is a diagrammatic elevation partly in section of one form of apparatus in accordance with the invention,

Fig. 2 is a fragmentary view of Fig. 1 with slight modification.

Fig. 3 is a View similar to Fig. 1 of another form ,of the invention, and

Fig. 4 a similar view of still another form. Referring firstto Fig. 1, a piston 2 is guided in tit a ortion of do a cylinder I and is connected by a suitable -answer w th. em a l s e i g ad ejte'st, piece is indicateduat .4 and the stationf's'tressing head at 5. pylinder l is; connected by .apipe fiwith apulsation device, which maycfonsist of a cylinder?! which a piston ii reciprocates. The piston; can. be moved, for example by means of. a swinging lever '9 actuated by a rotating eccentric Ill; The test piece 4 can be put'under a static initial stress, in. this case under a pressure, by means of a piston 12 guided in a cylinder H and loaded in any suitable way, for example, by means of compressed air. The meansfor applying-the initial stress must be elastic. Insteadof-the pneumatic device shown in Figb-l, it may forexample, be in the form of a piston 20 (Fig. 2) guided in a cylinder 2] and loaded by a spring 22. Now when the pulsation device is driven, there arise in the cylinder I varying pressures, which act more or less against the pressure in cylinder H, or against the pres- 5 sure of spring 22. Consequently, at every instant there is exerted on the test piece 4 only the difference between the forces exerted in the cylinders I. and H and this net force will be in one direction or the other according to the magnitude of the static initial stress and the limits within which the pressures exerted by the-pulsation device vary. In the example shown the test piece will accordingly be subjected to periodically changing tension and compression. stresses.

In order to indicate the forcesthere may be Provided manometers l3 and it, one to show the maximum load applied to the test piece in one direction and the other to show the maximum load applied to the test piece in the opposite direction. These manometers can be connected alternately to the cylinder l in the known way by means of a rotating piston valve l5 provided with ducts i511 and i517, which alternately open pipes E la and 13a to pipe 24 leading from the cylinder I. The valve if: can be driven by any suitable gearing diagrammatically indicated at 25, in unison with the eccentric Ill. The manometer M will be connected at the instant of minimum pressure to cylinder 1 and themanometer l3 at the instant of maximum pressure. The manometers l3 and M can be differential pressure measuring devices, in which case they will be connected by piping 256 with cylinder II. The cylinder H can be supplied with compressed air from a pipe 2'! controlled by a valve 28.

Preferably, the elastic transmitting device, such as the pneumatic device l l, l2 shown in Fig. 1, or the spring device shown in Fig. 2, will be so constructed that movement of the stressing head 3 under the alternating loads will cause only an inconsequential change in the static initial stress. This can be accomplished with ease when testing such inelastic materials as metals, because of the very short stroke of the piston l2 orZll permitted by the test piece, for instance just a few millimeters.

The apparatus shown in Figs. 1 and 2 has certain disadvantages. The spring device, in order to exert large forces. must have very large dimensions and is thereby rendered notsufficiently elastic for exerting small forces for which the same machine may be used. Furthermore, itis difficult to build it into the machine on account of the large space it requires. By the use of a pneumatic device there arises the difficulty that in exerting very high pressures of more than a hundred atmospheres it is necessary to load the device with compressed air, in order to get any sort of useful equalization Whatever. This is also unsatisfactory and expensive.

These difficulties are avoided to a considerable extent by connecting the initial stressing means (for instance, the expansible chamber filled with a compressed fluid), to an equalizing device serving as an elastic intermediate member. This latter device may consist of a spring loaded piston guided in a cylinder. Preferably, the spring loading means of the equalizing piston comprises a plurality of springs which can be adjusted, or brought into action only in part or all together, according to the magnitude of the alternating load. In this way the advantages are achieved, that, on the one hand, by means of the hydraulic transmission the forces acting on the springs can be made substantially smaller and, on the other hand, by the use of several adjustable springs their action can be adapted to the forces to be exerted for any particular purpose.

An example of this form of invention is shown in Fig. 3. The working cylinder of the machine is indicated at 3| and guides a piston 32 connected by a framework with the movable stressing head 33. The test piece is indicated at 34 and the stationary stressing head at 35. The cylinder Si is connected by a pipe 36 with a pulsator device comprising a cylinder 31, a piston 38 guided in the cylinder, and a mechanically operated crank drive 39 for reciprocating the piston. For exerting the initial stress, opposing the periodically varying forces acting on piston 32, there is provided a cylinder 40 in which is guided piston 4|, which is also connected with the support of the stressing head 33. The pressure in cylinder 40 can be produced by means of a pump 43 driving the fluid in through a pipe 42.

There is also connected with the cylinder 40 the equalizing device, which consists primarily of a cylinder 44 with a piston 45 guided therein. This piston is preferably in the form of a plunger piston and is provided with a guide rod 46 connected to its outer end. A spring device 41 acts upon the piston 45 and may consist of a considerable number of individual springs, for example 8, arranged around the piston. These can be adjusted by means of screws 48 for more or less stress, or to bring some of them entirely out of operation.

In order to retain the springs in position when withdrawn from the piston, the screws 48 are preferably provided with guide pins 49.

According to the forces to be exerted by the testing machine from time to time, by manipulation of the screws 48 more or less of the springs 41 are brought into operation and suitably stressed. Now under the action of the pulsating counterforces the cylinder 44 and piston 45, in combination with the springs 41' act as an elastic intermediate member, so that the movements of the stressing head 33 in accordance with the changing loads cause no change of the initial stress.

Another and preferred form of the invention is illustrated in Fig. 4. In order to provide an elastic transmission device which is only slightly altered in respect to its initial stress by movements of the stressing head, in accordance with the apparatus shown in Fig. 4 the elastic device is adapted to exert the force which supplies the initial tension through a very slightly compressi ble fluid confined within a closed vessel. The fluid, which may be oil, is in such quantity, and

the vessel is of such size and structure as to provide the required equalization.

On account of the relatively low compressibility of liquids they are known to offer the simplest means for transmitting large forces, while on the other hand, a suitably large quantity of liquid has sufiicient compressibility to bring about the above described equalizing, particularly with the assistance of the natural breathing of the vessel within which it is confined.

Fig. 4 shows an apparatus similar to Fig. 3 in respect to the main working cylinder and stressing heads, but the pipe 52 leading from cylinder 40 goes to a vessel 53, which is completely enclosed and which preferably is capable of slight expansion and contraction of its walls to assist in the equalization of the initial stress. The vessel 53, pipe 52, and cylinder 40 contain a fluid having very slight compressibility, such as oil. A pump 55 connected to pipe 52 by pipe 56 serves to control the pressure of the fluid in the system.

Having described my invention, what I claim is:

l. The method of testing materials, which comprises stressing the material with a definite static initial stress and intermittently relieving the material of the static stress and stressing the material oppositely by opposing to the static stress a greater pulsating stress directed oppositely to the static stress, thereby subjecting the material to a periodically changing net stress constantly equal to the varying difference between said stresses.

2. Testing apparatus for applying loads to a test piece alternately in opposite directions, comprising means for applying a definite static initial stress to the material, and pulsating means working directly against said first means and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress of said first means on the test piece and to apply an opposite stress to the test piece.

3. Testing apparatus for applying loads to a test piece alternately in opposite directions, comprising an elastic force exerting device for applying a definite static initial stress to the material, and pulsating means working directly against said first means and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress of said first means on the test piece and to apply an opposite stress to the test piece.

4. Testing apparatus for applying loads to a test piece alternately in opposite directions, comprising an elastic force exerting device for applying a definite static initial stress to the material, and pulsating means working directly against said first means and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress of said first means on the test piece and to apply an opposite stress to the test piece, said elastic force exerting device being so dimensioned that the movements of the stressing means cause only an inconsequential change of its initial stress.

5. Testing apparatus for applying loads to a test piece alternately in opposite directions, comprising means for applying a definite static initial stress to the material, pulsating means opposed to said first means for periodically reversing the stress, and a differential indicating device connected to said means so as to indicate net instantaneous stresses applied to the material.

6. In a testing apparatus for applying loads to a test piece alternately in opposite directions, a cylinder block containing two cylinders, pistons in said cylinders, a stressing head adapted to engage the test piece connected to one of said pistons, resilient means for exerting a substantially constant static force on said last mentioned piston to place the test piece under an initial stress in one direction, means for exerting pulsating forces on the other piston, and means for transmitting said pulsating forces to said stressing head in a direction opposed'to said static force.

7. Testing apparatus for applying loads to a test piece alternately in opposite directions, comprising means for applying a definite static initial stress to the material, and pulsating means working directly against said first means and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to sup press the stress of said first means on the test piece and to apply an opposite stress to the test piece, and a yielding equalizing device connected to said first means, adapted to keep the initial stress substantially constant in spite of movements of said stressing means.

8. In a testing apparatus for applying loads to a test piece alternately in opposite directions, means for applying a definite static initial stress to the material, comprising a fluid pressure device having an expansible chamber, and an equalizing device comprising a cylinder connected to said chamber and a spring loaded piston guided therein; and a pulsating device working directly against said fluid pressure device and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress exerted by said fluid pressure device on the test piece and to apply an opposite stress to the test piece. 7

9. In a testing apparatus for applying loads to a test piece alternately in opposite directions, means for applying a definite static initial stress to the material, comprising a fluid pressure device having an expansible chamber, and an equalizing device comprising a cylinder connected to said chamber and a spring loaded piston guided therein, said springs being adjustable in accordance with the magnitude of the alternating stresses applied to the test piece; and a pulsating device working directly against said fluid pressure device and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress exerted by said fluid pressure device on the test piece and to apply an opposite stress to the test piece.

10. In a testing apparatus for applying loads to a test piece alternately in opposite directions, means for applying a definite static initial stress to the material, comprising a fluid pressure device having an expansible chamber, and an equalizing device comprising a cylinder connected to said chamber, a spring loaded piston guided therein, and means whereby the number of said springs opposing the piston movement can be varied to conform to the magnitude of the stresses applied to the test piece; and a pulsating device working directly against said fluid pressure device and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress exerted by said fluid pressure device on the test piece and to apply an opposite stress to the test piece.

11. In a testing apparatus for applying loads to a test piece alternately in opposite directions, means for applying a definite static initial stress to the material comprising a closed vessel containing a very slightly compressible fluid through which the stresses are applied, and pulsating means working directly against said first means and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress of said first means on the test piece and to apply an opposite stress to the test piece, the quantity of the fluid and size and structure of said vessel being such that movement of said stressing means causes only an inconsequential alteration of the static initial stress 12. In a testing apparatus for applying loads to a test piece alternately in opposite directions, means for applying a definite static initial stress to the material comprising a fluid pressure device having an expansible chamber, and a vessel connected with said expansible chamber, said chamber and vessel being filled with a very slightly compressible fluid, and pulsating means working directly against said first means and exerting forces only in a direction opposite to the direction of said initial stress, in order periodically to suppress the stress of said first means on the test piece and to apply an opposite stress to the test piece, the quantity of the fluid and size and structure of said vessel being such that movement of said stressing means causes only an inconsequential alteration of the static initial stress.

WILHELM MARX. 

